U.S. Pat. No. 4,737,268 discloses a particle classification system in which a liquid stream containing different types of particles is introduced into an enclosed channel subjected to a field gradient that is transverse to the direction of flow. The different types of particles respond differently to the field and are segregated into different flow substreams within the channel according to the effect of the field on the particles. These substreams are intercepted at spatially displaced positions in order to recover the different particles.
This principle of particle separation would be useful in connection with liquid toner based electrophotographic copying machines for filtering charged toner particles from the toner liquid for these machines. In these types of copying machines, an electrostatic latent image is formed on a photoconductive carrier movable into proximity with a developer electrode held at a voltage intermediate the voltages on the carrier representative of background and information portions of the image. Liquid toner, comprising dielectric carrier liquid containing charged toner particles, is applied between the carrier and the developer electrode. As regions of the carrier associated with background portions of the image move past the developer electrode, the local electric field is directed toward the electrode and charged toner particles in the vicinity are drawn toward the developer electrode. As regions of the carrier associated with information portions of the image move past the electrode, toner particles in the vicinity are drawn to and are plated-out on such regions thereby developing the latent image on the carrier into a visible image.
After the image is developed, the carrier moves to a transfer station where the developed image is transferred to a receiving sheet. Thereafter, the carrier is cleared of any residual toner particles, charged to a high voltage at a charging station, and then moved to an image transfer station where another image is optically projected onto the carrier. The process described above then repeats.
It is often useful in liquid toner based electrophotographic copying machines to process the liquid toner to recover liquid carrier substantially free of toner particles. To this end, a separation system similar to that disclosed in the '268 patent referred above can be utilized. In such case, toner laden liquid toner would pass through a conduit that divides the flow into two separate flow paths just downstream of a pair of electrodes that establish a unidirectional field transverse to the direction of flow. If the toner is negatively charged, the electric field urges the particles towards the positive electrode increasing the concentration of particles in the fluid adjacent that electrode. Thus, a flow pattern is created which carries most of the particles that entered the conduit into the flow path adjacent the positive electrode. The remainder of the liquid flowing through the other of the flow paths will be substantially free of toner particles.
The basic problem with this arrangement is the plating-out of toner particles on the positive electrode which has a two-fold effect: a local neutralization of the electric field due to the presence of the negatively charged particles on the positive electrode, and a physical reduction in the flow path. Thus, a periodic deplating operation has to be carried out. While techniques for periodically deplating the positive electrode are available (e.g., see U.S. Pat. No. 4,168,329, the disclosure of which is hereby incorporated by reference), the necessity for this procedure adds to the complexity and cost of the equipment.
It is therefore an object of the present invention to provide a new and improved field assisted filter, and an electrophotographic copying machine using the same, which eliminates or substantially reduces the need for deplating the electrode of the filter.